This invention relates to a MRI magnet design. More specifically, it pertains to a magnet design having front and perpendicular oriented magnets, providing the middle cross-sectional area (i.e. in the air gap) with superimposed flux and obtaining, thereby, a high field strength along with a high field homogeneity starting right at the front edge of the magnet. The invention also relates to implementing a remote shimming concept, which makes possible mammography and imaging other external parts of body.
There are known whole body MRI systems (superconductive, permanent magnets and resistive magnets), having field homogeneity in the air gap, i.e. imaging volume, in the center of the magnet and, therefore, any part of the body has to be placed in the center of the magnet to be imaged.
However, the whole body magnets are still very expensive to purchase and maintain and, therefore, they are not widely available for the substantial need to provide a regular and frequent imaging like mammography as well as imaging the other organs. Prior art electromagnets such as described by Watson et al and Muller et al, and other prior art iron core magnets have a substantial edge fringe field effect, which makes it hard to image beginning immediately at the magnet edge or even proximal to the edge of the magnet due to lack of sufficient field homogeneity. In addition, those types of the prior art magnets, of the iron core electro- or permanent type, typically have a structural design to provide a high strength magnetic field at a large central air gap.
Accordingly, an object of this invention is to provide a method and apparatus to improve the field homogeneity at front edge of the magnet, and increase the volumetric field strength by a magnet configuration providing a flux superposition from two independent flux sources, evenly contributing flux into the same air gap.
A basic magnet configuration of this invention can be made using two types of the flux generation sources: permanent magnets with magnetization (or demagnetization) coils and electromagnets, which in some cases can be superconductive.
Other objects, advantages and features of the invention will become apparent from the following specification and appended claims when taken with the accompanying drawings.